Wikipedia mentions a required "availability search" to find peers (and the actual file):
Note that, although a particular file is indicated, an availability search for it must still be carried out by the client application.
Where does the client look? Does a magnet link require a tracker URI or is that up to the client's network?
A certain magnet URI/URN from tpb looks like this:
It contains 4
tr query params with (I suppose) tracker locations that contain some sort of hash index.
However, Wikipedia doesn't mention the
tr param, so I assume it's not mandatory.
Where does a client start looking for the file if no tracker URI's are included? And if there are? I can imagine a torrent client (like uTorrent) itself having an enormous index of file hashes.